1. Field of the Invention
The invention relates to the formation of felts which are formed from mineral fibers and referred to by the generic names of glass wool, rock wool, etc.
2. Discussion of the Related Art
Conventionally, mineral fiber felts are made continuously by depositing the fibers on a conveyor, these being conveyed by gas streams. The conveyor retains the fibers and allows the gases to pass through them.
Before they are deposited on the conveyor, the fibers are coated with a composition applied in liquid form and intended to bond the fibers together, so as to give the subsequently formed felt its cohesion. Composition is then crosslinked by a heat treatment carried out on the felt, which has been conditioned beforehand to the desired thickness and density.
In addition to the insulating properties required very generally, it is also sometimes necessary for the products used to have very specific mechanical properties. This is the case, for example, with products which support masonry elements and which consequently have to withstand large compressive forces, such as elements used for the insulation of flat roofs. This is also the case with products used as exterior insulation which must in particular be able to withstand tearing forces.
To obtain products having these particular properties and other which will be seen later, it is necessary to modify the conventional processes for forming felts.
Forming felts by depositing fibers on the receiving conveyor or on a similar member results in an entanglement which is not uniform in all directions. Experimentally, it has been found that the fibers have a strong tendency to lie parallel to the receiving surface. This tendency is all the more accentuated the longer the fibers.
This structure of the felts is favorable to their insulating properties and to their tensile strength in the longitudinal direction. Consequently, such a structure is advantageous for many uses. However, it will be understood that such a structure is not best suited when, for example, the product has to withstand compression or tearing in its thickness direction.
To improve the compressive strength of these felts, one solution consists in increasing their density, by increasing the mass of fibers per unit surface on the receiving member where the felt is formed. Besides the fact that the mass of fibers per unit area that can be deposited is limited, the build-up of fibers on the receiving member rapidly preventing the flow of gases and therefore preventing the felt from continuing to be formed properly, which does not allow other properties such as the tear resistance to be improved.
Another solution already proposed consists in ensuring that the direction of the fibers lies not in the plane of the felt but in a plane perpendicular to it. This arrangement is achieved, for example, by forming pleats in the felt. These pleats are in particular obtained either by placing the felt in successive layers of greater or lesser length lying in the desired final thickness direction or by compressing the felt longitudinally. Due to the effect of the compression under the conditions envisaged, the felt forms corrugations. The heat treatment of the binder composition, which is carried out subsequently, gives this pleated structure permanence.
The direction of the fibers oriented in the thickness direction of the felt thus formed makes it possible to substantially improve the compressive strength and the tear strength. However, this structure is to the detriment of the longitudinal tensile strength—the felt has a tendency to unfold—or to the flexural strength.
The arrangement of the fibers in the thickness may also result from the assembly of strips of felt whose width corresponds to the desired thickness of the felt, each strip being placed so that the fibers lie in planes perpendicular to the faces of the felt formed. The strips are held against each other by means of a coating or a film covering one or both faces of the felt. Optionally, the strips may also be adhesively bonded directly to one another via their contacting surfaces.
The felts produced using this relatively complex technique, called “strip-webs”, are used mainly for insulating large-diameter pipes. For this application, instead of constituting a disadvantage, the ability of the product obtained to bend and even to be wound is particularly desirable.
One solution to this problem is described in patent EP 0 133 083 filed in the name of the Applicant. The aim of this prior invention is to provide felts in which the mechanical properties, especially the compressive strength and tear strength in the thickness direction of the product, are improved without incurring the drawbacks encountered previously and consequently without forming pleats or assembling strips of felts; moreover, the mechanical and insulating properties of said felts remain satisfactory.
To do this, attempts have been made to obtain fibers (within the felt) having directions as varied as possible without correspondingly forming surface pleats. According to this prior art, the felt is subjected to at least one longitudinal compression operation and preferably to two operations of this type.
One problem associated with such manufacturing techniques resides in the homogeneity of the final felt and in particular in its surface homogeneity. This is because cracks, fracture initiators and/or localized swellings are observed on the surface, these being defects which are negative in terms of the mechanical and thermal behavior, without mentioning the unattractive appearance of the assembly.
This problem is essentially created in the region between the conveyor belts and the crosslinking oven.
In this region, the felt is no longer compressed so that it tends to loosen.
The aforementioned patent provides slideways in this region so as to ensure a certain continuity in holding the felt. However, the slideways are then additional and fragile elements, the more so as they are placed in the immediate vicinity of the oven, which is hot. Moreover, they must be positioned perfectly so as to ensure the desired continuity. In fact, this solution is not viable and the slideways pose more problems than they solve; this is why in practice this solution has been abandoned in favor of another solution, which constitutes the subject-matter of the present application.